In general, image monitoring systems have a function of capturing an image from a camera, and detecting a moving object such as a human or a vehicle which appears in a monitored area by image recognition. By utilizing the result of the moving object detection, these image monitoring systems are capable of recording only an image in which a moving object appears, causing a display unit to display an alert icon, or sounding a buzzer to alert a guard. Therefore, they are useful for reducing a burden on watching service in which continuous check used to be required.
Further, the above-mentioned image monitoring systems bring the following advantage: when a crime, such as theft, occurs, it is possible to make good use of a recorded image to thereafter evidence the crime.
Meanwhile, criminals perform various acts on such image monitoring systems in an attempt to prevent an evidential image from being acquired, and this has become problematic. Such acts include changing the image pickup angle of a camera, placing an obstruction in front of a camera, and destroying a camera. Incidentally, in case that the imaging device itself falls in faulty, an image cannot be acquired and there is a possibility that the image monitoring system becomes useless. In this case, it is required to notify a responsible guard or the like of this anomaly in the equipment and repair the camera to restore the image monitoring system to a normal state.
As a method for coping with this problem, there is known the technique disclosed in JP-A-2000-222646. This method is such that a normal image of reference is held and an inputted image is compared with this image to detect any anomaly in a camera.
The method disclosed in JP-A-2000-222646 involves a problem. This method is based on amount of change in the brightness value of images as follows. In this method, first, determining is the ratio of pixels where a brightness difference value between the reference image and an inputted image varies by more than a predetermined value. When this ratio exceeds a predetermined value, it is determined that sabotage against the camera has been done and this is notified. This construction involves a problem. When sabotage is obvious, for example, an obstruction is placed or an angle of camera view is changed, an alert can be put out. However, in an event in which the situation is difficult to estimate, for example, when a pickup image blur occurs in a lens, there is a possibility that the system fails to put out an alert. (“Blur” cited in this specification refers to a state in which an image is not clear and bleary.) Conversely, when normal change, such as change in lighting conditions and a shift in the weather, occur, there is a possibility that erroneous alerts are frequently put out.
The invention has been made to solve the above problems. The invention is to provide an image monitoring system in which any anomaly in a camera can be properly detected even when an image blur, change in lighting conditions, or a shift in the weather occurs.